It is known (R. E. Kaiser et al., Rapid Commun. Mass Spectrom. 3, 225 (1989), R. E. Kaiser et al., Int. J. Mass Spectrom. Ion Processes 106, 79 (1991)) how to eject the ions mass-sequentially by a fixed dipole alternating field while slowly increasing the amplitude of the storage radio frequency linearly. The dipole alternating field is generated by an alternating voltage applied to the two end caps of the ion trap. The ions leave the ion trap through a perforated end cap and can be detected outside the trap with conventional means. The method is particularly used for ions of very high masses in the range from approximately 5,000 u to 50,000 u.
If ions with different mass-to-charge ratios are stored in an RF quadrupole ion trap according to Wolfgang Paul and Helmut Steinwedel (U.S. Pat. No. 2,939,952), they can, according to present knowledge, be ejected mass-selectively, i.e. temporally separated in the order of their mass-to-charge ratios, by three different methods in an axial direction through one of the two end caps and detected outside in the form of a mass spectrum. For reasons of simplicity, only masses and not mass-to-charge ratios are referred to in the following. Although, strictly speaking, this applies only to singly charged ions, it should not be understood in a restricted sense here. The three mass selective ejection methods are as follows:
(I) The "mass selective instability scan" (U.S. Pat. No. 4,540,884) uses the stability limit .beta..sub.z =1 of the first stability region in Mathieu's stability diagram. (See the following relevant literature: P. H. Dawson, "Quadrupole Mass Spectrometry and its Applications", Elsevier, Amsterdam, 1976; and R. E. March and R. J. Hughes, "Quadrupole Storage Mass Spectrometry", John Wiley & Sons, New York 1989). The working points of the ions are shifted across the stability border .beta..sub.z =1 by a continuous change in the operating parameters of the ion trap. To do so, the RF voltage of the storage field, the so-called drive voltage of the ion trap, is preferably enlarged linearly, this operating method resulting in a linear mass scale. The ions becoming instable according to their order of mass enlarge their oscillation amplitude in the axial direction ("z" direction) on the other side of the stability border by the absorption of energy from the storage RF field in a temporally exponential manner and are finally able to leave the storage space of the ion trap through perforations in one of the end caps. Given certain conditions for the precise form of the quadrupole field (U.S. Pat. No. 5,028,777), this method provides spectra with a good mass resolution, i.e. ions of one mass are fully ejected and can be completely measured before it is the turn of ions of the next mass.
(II) The "scan by nonlinear resonances" (U.S. Pat. No. 4,818,869 and U.S. Pat. No. 4,975,577) uses the amplitude growth, which our findings show to be sharply hyperbolic, in the secular oscillations due to nonlinear resonance conditions which arise in the ion trap due to superposition of the quadrupole field with higher-order multipole fields. As a result of the hyperbolic amplitude growth in the nonlinear resonance, this method leads to particularly quick scanning with a good mass resolution. Since the multipole fields at the center of the ion trap disappear, ions resting at the center after cooling with a collision gas are unable to experience the nonlinear resonances. They therefore need to be pushed through a dipole alternating field, the frequency of which is the same as or a little lower than the resonance frequency. The mass flow is generated as in method (I) by changing the operating parameters of the ion trap, preferably by a linear change in its drive voltage.
(III) In addition, the ions can be expelled from the ion trap by resonant dipolar excitation in the axial direction. The dipole field is generated by an alternating voltage which is applied between the two end caps. Initial applications of the method are known from as long ago as the 1950s. A detailed description of the various ejection options is given in U.S. Pat. No. Re. 34,000 (reissue of U.S. Pat. No. 4,736,101). The most successful method is to leave the frequency of the alternating voltage applied at the end caps for generation of the dipole field constant and to linearly increase the drive voltage of the ion trap. This causes the ions to undergo a change in the frequency of their secular oscillations. If the secular oscillations of a mass's ions enter into resonance with the dipole alternating field in the z-direction, the ion oscillations absorb energy from the dipole alternating field and enlarge their oscillation amplitude, enabling them to leave the ion trap if the dipole alternating field is sufficiently strong.
Method (I) cannot be used for the ions of very high masses exceeding approximately 5,000 atomic units of mass u since the RF voltage is limited to approximately 15 kV by practical ion trap requirements such as gas pressure in the ion trap and insulation distances. A collision gas pressure of approximately 10.sup.-3 millibars must normally be maintained in ion traps. With the limitation to approximately 15 kV and a minimum frequency of approximately 500 kHz, which depends on the required number of storable ions, conventional ion traps have a resulting upper limit of approximately 4,000 u for the practically usable mass range.
The mass range of method (II) is only marginally higher since the effective nonlinear resonances are not very far away from the instability limit. The most effective resonance at the point .beta..sub.z =2/3 of the hexapole field is only approximately 12% higher in mass than the stability limit .beta..sub.z =1, related to the same RF voltage. All higher nonlinear resonances (from approximately .beta..sub.z &lt;1/2) cannot be used for this method since they are far too weak.
For this reason, method (III) has so far been used for ions of very high masses in the range of some 10,000 unified atomic mass units u (R. E. Kaiser et al., Rapid Commun. Mass Spectrom. 3, 225 (1989), R. E. Kaiser et al., Int. J. Mass Spectrom. Ion Processes 106, 79 (1991)). The method has, however, a serious drawback: it is extremely slow. In the papers above, approximately 500 secular oscillations were required for ejection of the ions of a mass to achieve a single mass resolution just resulting in the separation of two adjacent masses. The scan speed for this single mass resolution (not a high resolution) must therefore not exceed one mass unit for every 500 secular oscillations. In this regard, it must be taken into account that the secular oscillations of the heavy ions are very slow. (In the .beta..sub.z &lt;0.6 range, the secular oscillation frequencies .omega..sub.z are approximately inversely proportional to the mass). In comparison, ions of one mass can be completely ejected in approximately 10 secular oscillations with method (II), while commercial equipment working according to method (I) uses a scan speed of approximately one mass per 90 secular oscillations. With a dipole alternating frequency of 25 kilohertz, method (III) provides a scan speed of only 50 mass units per second, while method (II) measures 30,000 mass units per second, though at 333 kilohertz in the lower mass range.
Physical Principles
Our most recent examinations have shown that the increase in amplitude of the secular oscillation in a resonant alternating field depends on the multipole ordinal number of the exciting alternating field. It can be shown that the following differential equation holds for the temporal increase in amplitude in the z-direction: EQU dz/dt=C.sub.n *z.sup.(n-1), n=multipole order. (1)
Integration produces the following: EQU z.sub.1 (t)=C'.sub.1 *t a linear increase for the dipole (n=1), (2) EQU z.sub.2 (t)=C'.sub.2 *exp(t) an exponential increase for the quadrupole (n=2), (3) EQU z.sub.3 (t)=C'.sub.3 /(t-C".sub.3) a hyperbolic increase for the hexapole (n=3). (4)
Equations (2), (3), and (4) have been verified by computer simulations. Equation (2) was simulated by an electrical voltage at the end caps, equation (3) tested by means of the increase in amplitude at a fixed working point in the instable range, and equation (4) at different nonlinear resonances of superposition with a hexapole field which was generated by the shape of the electrodes. FIGS. 1 to 3 show the results of the computer simulations.
It can be expected from these examinations that equation (3) with an exponential rise in the secular oscillation amplitude also applies to the case of superposition with a resonant quadrupole alternating field generated by electrical means with an alternating voltage between the ring and end cap electrodes.
Therefore, it is among the objects of the invention to specify a fast scan method for the spectra of ions in a quadrupole ion trap, which, in particular, can be used for ions of very high masses.